Interspinous Spacer and Facet Joint Fixation Device

ABSTRACT

A biologically implantable device including a spacer body and fasteners for fixing the spacer body in place between vertebrae, and a method for use of the device to improve intervertebral spacing and correct foraminal stenosis, and to immobilize facet joints and promote fusion of adjacent vertebrae to each other. A drill guide may be used in providing bores in the facet joints to receive the fasteners.

TECHNICAL FIELD

The present invention relates to spinal surgery, and in particularrelates to an implantable device and a procedure for use of such adevice in fusing vertebrae together.

BACKGROUND ART

Lower back pain is commonly caused by deterioration of intervertebraldiscs and facet joints, resulting in stenosis of intervertebral foraminaand thus causing pressure on spinal nerve roots where they exit from thespinal nerve canal and extend laterally away from the spine through theintervertebral foramina. As people age, intervertebral discs and otherparts of the spinal column deteriorate. Discs may shrink, allowingvertebrae to approach one another more closely, and facet joints maydevelop bone spurs, protruding into the foramina and reducing theavailable space through which nerve roots must extend. Pressure on nerveroots due to facet arthrosis, ligamentum flavum infolding, and discherniations often results and can cause severe and even debilitatingpain. Degeneration of discs may also allow a vertebra to becomemisaligned with respect to an adjacent vertebra, resulting inspondylolisthesis.

It is well known to decompress the spine, that is, to relieve pressureon the spinal exiting and traversing nerves by performing bilaterallaminotomies and to immobilize adjacent vertebrae with respect to eachother. Installing pedicle screws and rods can also be used to stabilizethe spine if necessary.

It is also known that facet joints can be immobilized, and that a lumbarvertebra can be fused together with an adjacent lumbar vertebra toprevent the vertebrae from moving relative to each other, in order toprevent future impingement on spinal nerves between the fused vertebrae.In the past, however, vertebral fusion has typically been accomplishedby the use of pedicle screws and rods to immobilize vertebrae relativeto each other. Compared with facet fixation, pedicle screws and rodinsertions requires a longer operative time, and results in increasedblood loss, and increased paraspinal muscle trauma. Since spinaldeterioration of the sort which requires such surgical remedy usuallyoccurs in older patients, it would be desirable to be able to immobilizeand fuse vertebrae in a procedure that takes less time, less anesthesia,less blood loss, and less soft tissue trauma.

Recently, less invasive procedures have been utilized, such aspercutaneous installation of screws into facet joints to immobilizethem, as disclosed, for example, in Serhan et al. U.S. PatentApplication Publication Document No. US2005/0149030-A1. It is also knownto insert a long screw through a spinous process and into a facet joint,as shown in Obenchain et al. U.S. Patent Application PublicationDocument No. US2004/0254575-A1.

Many different methods and devices are known for use in immobilizingvertebrae relative to one another to promote fusion. One type of deviceintended for immobilizing adjacent lumbar vertebrae with respect to eachother includes a spinous process fixation system in which a clamp isfastened to the spinous processes of two adjacent vertebrae, fasteningthem together. Such devices are disclosed in Chin et al. U.S. PatentApplication Publication Document No. US2007/0270840-A1 and Lamborne etal. U.S. Patent Application Publication Document No. US2008/0183211-A1.Such clamps can be attached so as to distract the vertebrae from eachother, but because of the location of such a clamp, spaced apartdorsally from the deteriorated intervertebral discs, use of such a clampmay result in reduced lordosis, thereby resulting in additional strainexperienced by joints between other vertebrae. Such clamp devices,moreover, often cannot be used in an L5-S1 location, because of the lackof a large enough spinous process on the sacrum. Additionally, theinterconnected spinous processes may fail under the forces appliedthrough such clamps.

What is desired, then, is a device and a method for its use in relievingpressure on spinal nerves and in fusing vertebrae together, using aminimally invasive procedure that can be performed in a significantlyshorter time than has previously been required for vertebral fusionprocedures such as those including the installation of pedicle screwsand rods.

DISCLOSURE OF INVENTION

The present invention provides answers to some of the above-describedneeds and shortcomings of previously known devices and procedures, asdefined by the claims appended hereto.

According to a principal aspect of the present invention, an implantabledevice includes a spacer body that can be implanted between the spinousprocesses and between the articulating processes of a pair of adjacentvertebrae that are to be fused together. The spacer body supports thevertebrae in a desired relationship. A pair of fasteners such as screwsextend through the spacer body, with each screw extending also throughthe facet joint on a respective lateral side of the spine so as toimmobilize the facet joint and promote fusion between thecephalad/caudad lamina, spinous process, and facet joints.

As an aspect of one embodiment of the implantable device disclosedherein, locking devices are provided in the spacer body to ensure thatfasteners installed to hold the spacer device in place and to immobilizefacet joints are kept in place relative to the spacer body.

Another aspect of the device and procedure disclosed herein is that adrill guide may be used in conjunction with the spacer body to direct adrill or similar tool used to prepare a hole in the facet joint andaligned with a fastener receptacle in the spacer body to receive afastener to immobilize the facet joint on each lateral side of thelocation of the spacer body.

The foregoing and other features of the invention will be more readilyunderstood upon consideration of the following detailed description ofthe invention taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side elevational view of a lower portion of a human spine inwhich an intervertebral disc has deteriorated.

FIG. 2 is a view similar to FIG. 1, showing the same portion of a spineafter performance of a surgical procedure as disclosed herein to fusetwo vertebrae together.

FIG. 3 is a view taken in the direction indicated by the line 3-3 inFIG. 2, showing a portion of the spine shown in FIG. 2 surgicallyexposed for performance of corrective surgery in accordance with thepresent invention.

FIG. 4 is a view similar to FIG. 3, showing the portion of a spine aftercompletion of a bilateral laminotomy on the cranial one of the exposedvertebrae.

FIG. 5 is a view similar to FIG. 4, showing a vertebral fusion-promotingspacer body in place between the spinous processes, which are shownpartially cut away for clarity.

FIG. 6 is a view similar to FIG. 5, showing a vertebral fusion-promotingspacer and facet joint fixation device installed in the exposed portionof the spine.

FIG. 7 is a sectional view taken on line 7-7 of FIG. 6.

FIG. 8 is a partially broken away view similar to a portion of FIG. 2,at an enlarged scale.

FIG. 9 is an isometric view of a drill guide for use in connection withimplantation of the fusion-promoting spacer body.

FIG. 10 is an isometric view of the interspinous process spacer bodyportion of the fusion-promoting device, at an enlarged scale.

FIG. 11 is an elevational view showing the inferior bone-engaging faceof the spacer body, shown in FIG. 10.

FIG. 12 is a plan view taken from the dorsal, or upper, side of thespacer body as it is shown in FIG. 10.

FIG. 13 is a view of the device shown in FIG. 10, taken from theventral, or lower side as the spacer body is shown in FIG. 10.

FIG. 14 is an end elevational view, showing the right lateral end faceof the spacer body shown in FIG. 10.

FIG. 15 is an elevational view of the superior vertebra-engaging face ofthe spacer body, showing an alternative non-slip textured surfaceconfiguration for the face.

FIG. 16 is a sectional view through the spacer body, taken along line13-13 of FIG. 9.

DETAILED DESCRIPTION OF MODES FOR CARRYING OUT THE INVENTION

Referring now to the drawings, which form a part of the disclosureherein, in FIG. 1 a part of a spinal column 20 is seen in profile fromthe right lateral side. The illustrated part of the spinal columnincludes a sacrum S1 and lumbar vertebrae L5, L4, and L3. Intervertebraldiscs 22, 24, and 26 separate the vertebrae from one another and, in ahealthy individual, maintain sufficient spacing between adjacentvertebrae.

In the spinal column 20 as shown in FIG. 1, however, the disc 24 betweenthe L4 and L5 vertebrae has deteriorated and has allowed the spacing 28between the L4 and L5 vertebrae to decrease markedly from normalspacing. The L4 vertebra has also been able to move ventrally withrespect to the L5 vertebra, and the disc 24 is herniated and has bulgedin a dorsal direction, as shown at 30. The superior articulating process34 of the L5 vertebra has become hypertrophied, and an osteophyte, orbone spur 32 protrudes from it toward the nerve root 38.

The combination of these defects or abnormalities in the L4-L5 locationcauses a stenosis, or reduction of the size, of the foramen 36, that canresult in pressure on the spinal nerve 38 where it exits from the spinalcolumn between the L4 and L5 vertebrae. The spinous process 40 of the L4vertebra is shown in FIG. 1 to have approached the spinous process 42 ofthe L5 vertebra, so that the vertical distance 44 between the spinousprocesses 40 and 42 has been significantly reduced from normal.

As shown in FIG. 2 the spinal column 20 has been surgically repaired.The L4 and L5 vertebrae have been moved apart from one another and areheld in a more desirable relationship to each other by the implantationof an interspinous process spacer body 50 located between the spinousprocesses 40 and 42 to increase the vertical distance 44. Screws 52 aremounted in the spacer body 50 and extend into the right facet joint 54and the left facet joint (not seen), fastening together and immobilizingthe superior articulating process 34 of the L5 vertebra and the inferiorarticulating process 56 of the L4 vertebra, and the corresponding partsof the left facet joint.

As shown in FIGS. 3-6, the spacer body 50 and a pair of fasteners suchas screws 52 are implanted surgically through an incision 57 in apatient's back in the vicinity of the vertebrae (L4 and L5, as shown)that need to be fused together. The incision may be made along themidline from about the middle of the spinous process 40 of the cranialone of the affected vertebrae to about the middle of the spinous process42 of the caudal one of the affected vertebrae. As shown in FIG. 3, theincision is made along a medial line over the spinous processes 40 and42, after which skin, connective tissue, and muscle are retractedlaterally by retractors 58 and 60.

The spinous processes 40 and 42 are exposed, the interspinous ligamentis excised, and a bilateral hemilaminotomy may be performed on thecranial one of the vertebrae, if necessary, to enlarge the space 62between the affected vertebrae, in this case the L4 and L5 vertebrae asshown in FIG. 4. Laminotomy may not be required at all, or may berequired for both vertebrae involved, as may be determined by thesurgeon. Although the caudally located vertebra L5 may undergo alaminotomy, in most cases laminotomy will not be required in the caudalone of a pair of vertebrae to be fused together. The ligamentum flavumis removed from the space 62 between the vertebrae and exposed surfacesof the remaining parts of the laminae 63 may be partially decorticatedto enhance in-growth of bone tissue from the L4 and L5 vertebrae inorder to fuse the two vertebrae together into a rigidly unifiedstructure.

After completion of bilateral hemilaminotomy and removal of ligamentumflavum the enlarged foramen 36 is examined to ascertain that the exitingnerve root and the traversing nerve root have been decompressed and areclearly free of impingement of the remaining portions of the laminae 63and the facet joints 54 and 72 of the vertebrae concerned.

Referring next to FIG. 5, the surgeon selects from a set of similarspacer bodies of different sizes a spacer body 50 of what appears to bean appropriate size and inserts it, between the cranially locatedspinous process 40 and the caudally located spinous process 42, into thespace 62 defined between the remaining portions of the laminae 63 of thevertebrae L4 and L5, to determine that the selected spacer body 50 willprovide the desired height 64, as shown in FIG. 4, and to ensure thatthe selected spacer body 50 correctly fits within the width 66 of thespace between the inferior articulating processes 56 and 68 of the L4vertebra, the cranially-located one of the pair of vertebrae to be fusedtogether. The spacer body 50 of the correct size should fit snugly buteasily into that space, maintaining the increased height 64 betweenvertebrae L4 and L5 to compensate for the collapsed intervertebral disc22. When the correct size has been determined, the appropriate spacerbody 50 is inserted between the spinous processes 40 and 42 and fittedinto the space between the vertebrae concerned, to enlarge the verticaldistance 44 and the height 64 and preserve an enlarged space 62.

Once the spacer body 50 has been installed, it is necessary to providean opening to receive a fastener 52 to fix together and promote fusionof the superior and inferior articulating processes. While this can bedone by an experienced surgeon without mechanical guidance, it ispreferred to use a tool such as the drill guide 112 shown in FIGS. 7 and9 to bore a fastener hole through the facet joint 54 or 72 in an optimallocation and direction. Additionally, the use of a drill guide 112 canassist in determining the required length for a fastener 52 to beinserted through the spacer body 50 into each of the facet joints 54 and72.

The drill guide 112 may include a handle 114 attached to a drill guidetube 116 whose distal end 118 may be shaped appropriately to fit againsta selected portion of the fastener receptacle 88 or 90 of the spacerbody 50. A drill 120 may be provided specifically for use with the drillguide 112 and may be marked with graduations 122 along its shank toprovide a direct indication of the distance by which the drill 120extends beyond the distal end 118, or to provide a direct indication ofan appropriate length of screw 52 to be used, based upon the finalposition of the drill 120 when it has proceeded through both theinferior articulating process 68 of the cranial vertebra and thesuperior articulating process 74 of the caudal (L5) vertebra.

Spaced apart from the drill guide tube 116 by a spacer member 126 of anappropriate size is a feeler guide tube 128 in which a feeler probe 130is slidably carried. A distal end portion 132 of the feeler 130 may bebent at an appropriate angle to fit against a lateral aspect of asuperior articulating process 74, in order to assure that the guide tube116 is oriented as required for the drill 120 to form a bore in anappropriate location passing through the articulating processes 56 and34 or 68 and 74. Instead of, or in addition to the scale graduations 122that may be provided on a drill 120 for use with the drill guide 112, ascale of graduation marks 134 may be provided on the shank of the feeler130 to provide an indication of the distance between the distal end 136of the feeler guide tube 128 and the tip portion 132 of the feeler 130,as an indication of an appropriate length for a screw 52 to be used withthe spacer body 50.

Once the screws 52 or other appropriate fasteners have been inserted andtightened, they may be retained and prevented from loosening withrespect to the spacer body 50 by locking devices 92 that may be mountedin the fastener receptacles 88 and 90 in the spacer body 50, as shown inFIGS. 5, 7, and 16.

As may be seen in FIGS. 10-16, the intervertebral spacer body 50 may bea generally “H”-shaped or bowtie-shaped body with a pair of oppositegenerally concave sides, a first, bone-engagement or cranial vertebraengagement face 76 and a second bone-engagement, or caudal vertebraengagement face 78. Each of these faces may include a non-slip surfaceshape or texture, such as the narrow parallel grooves 80 extending alongthe caudal bone engaging face 78 as may be seen best in FIG. 10 and FIG.11. Opposite lateral end faces, a left end face 82 and a mirror-oppositeright end face 84, shown in FIG. 14, extend between the bone engagementfaces 76 and 78. The lateral end faces 82 and 84 may conveniently bemade flat, but may also be otherwise shaped, such as being convexlycurved surfaces, depending on how the spacer 50 is made. A dorsal face86 may be generally flat, as shown, but its precise shape also is notcritical, and depending on how the spacer body 50 is manufactured thedorsal face 86 may instead be generally flat or convexly rounded and mayhave any desired surface texture or may be treated or coated with aknown material to promote adhesion of new bone growth to incorporate thespacer body 50 into the vertebrae being fused together.

Extending into the dorsal face 86 at an acute angle and proceedingdiagonally toward the left and right end faces 82 and 84 are a pair offastener receptacles 88 and 90, each opening laterally outward through arespective one of the end faces 82 and 84. The fastener receptacles 88and 90 are formed to provide a secure purchase for the fastener tosecure the spacer body 50 yet permit some angular variation of thegenerally ventrally inclined orientation of a fastener 52 where itextends laterally outwardly from one of the lateral end faces 82 and 84.

Each fastener receptacle preferably includes a fastener locking device92 that is fixedly attached to the spacer body 50 to keep a fastenersuch as a bone screw 52 fitted in the fastener receptacle 88 or 90 fromloosening and backing out of the desired position once it has beeninstalled as shown above in FIG. 6. Such a fastener locking device 92may be of any desired functionally operable type. For example, a lockingdevice may include a spring 94 captured within a radially inwardly openchannel defined by a shell 96 as shown in FIG. 16, with the spring 94arranged to receive and lock around a radially-extending ridge 98 on ascrew 100 as shown in FIG. 7.

As shown in FIGS. 13 and 14, a recessed portion including a face 102 isdefined by a ledge face 104 that extends transversely across the ventralface 106 of the spacer body 50 so that the recessed face 102 can restupon the prepared dorsal surface of the lamina 63 of the cranial (L4)vertebra as shown in FIGS. 5, 6, and 8, with the end faces 82 and 84 ofthe spacer body 50 snugly located between the inferior articulatingprocesses 56 and 68. The portion defining the ventral face 106 protrudesin a downward direction as seen in FIG. 10, or a ventral direction inuse, to allow the body 50 to extend into the space 62 between thevertebrae L4 and L5. As shown in broken line in FIG. 13, a ledge face104 a having the shape of a partial cylinder might be provided insteadof the planar ledge face 104 shown in solid line. Such a ledge face 104a would fit more closely against the surface of the lamina 63 of thecranial (L5) vertebra when the spacer body 50 is in place between the L5and L4 vertebrae as shown in FIGS. 5-8.

As shown in FIG. 15, the surface of the cranial bone engagement face 76may have alternatively a non-slip surface shape such as a checkered, orknurled surface including intersecting patterns of parallel grooves andridges, and the ledge face 104 may also have such a surface texture, ormay simply be smooth.

The spacer body 50 should be of a biologically implantable material.Thus it may be fashioned of cortical bone machined to the desired shape,or may be made of a machineable or castable plastics material such aspolyether ether ketone (PEEK) or titanium. Similarly, the fastenerlocking devices 92 must be of a biologically implantable material,preferably a substantially biologically inert metal such as titanium.Also, the fasteners such as screws 52 should be of such a biologicallyimplantable metal such as titanium or another metal unlikely to reactwith body fluids or interfere with use of diagnostic equipment such asmagnetic resonance imagers.

Although various types of fasteners might be used, a screw 52 as shownin FIG. 7, including a coarse-threaded distal portion 108 and arelatively fine threaded proximal portion 110, advantageously engagesthe inferior articulating process 68 or 56 of the more cranial vertebra(L4 vertebra), and also tends to pull toward it the superiorarticulating process 74 or 34 of the more caudal vertebra (L5) when thescrew 52 is tightened.

Once the screws 52 have been installed in the spacer body so as toextend through the facet joints on both sides, the facet joints arepreferably exposed surgically and the bones of the articulatingprocesses 34, 56, 68 and 74 may be decorticated to a depth of about onemillimeter to promote growth of new bone to effect fusion. Additionally,a groove 140 may be fashioned by grinding away a small amount of thecartilaginous endplates to provide a place for insertion of afusion-promoting material such as a local bone autograft, allograftbone, or bone extenders such as demineralized bone matrix material orbone morphogenic protein to encourage growth of new bone interconnectingthe now-immobilized facet joints. Similar material 142 may be placed onand around the spacer body 50 and packed between the spacer body 50 andthe decorticated spinous process and lamina surfaces of the vertebrae tobe fused together, forming a matrix for regeneration of bone to fusetogether the vertebrae in the position established by the spacer body 50and the screws 52 in the facet joints. The spinous processes 40 and 42are held separated from each other at the appropriate distance 44 by thepresence of the spacer body 50, which is held in place by the screws 52that immobilize the facet joints.

As shown in FIGS. 6 and 8, such fusion-promoting material 142 can beutilized to fill voids between the spacer body 50 and the vertebraebeing fused. After installation of said fusion-promoting material, thesoft tissues that had been retracted are replaced around the vertebraebeing fused, so that they can provide mechanical support for thefusion-promoting gel or putty-like bone matrix and provide a supply ofblood, to the extent it is not provided by the remaining cancellous bonestructure, as the bones heal and fuse themselves together. The wound isthen closed in the same manner as for previously utilized procedures.

Because it is unnecessary to fully expose the lateral aspect of eitherof the facet joints 54 and 72, the size of the opening required for theprocedure described herein is significantly less than that required forinstallation of rods supported by pedicle screws or for installation ofscrews into the facet joints from a lateral aspect of such a joint. As aresult, the amount of blood loss and pain is less than in suchpreviously used procedures. Additionally, the procedure of installationof the spacer body 50 and screws 52 is less complicated, and theprocedure can therefore be performed in a significantly shorter time,thus requiring shorter general anesthesia and reducing the amount oftrauma suffered by the patient.

The terms and expressions which have been employed in the foregoingspecification are used therein as terms of description and not oflimitation, and there is no intention in the use of such terms andexpressions of excluding equivalents of the features shown and describedor portions thereof, it being recognized that the scope of the inventionis defined and limited only by the claims which follow.

What is claimed is:
 1. A biologically implantable device for use infusing together a pair of adjacent vertebrae, the device comprising: (a)a biologically implantable spacer body having a concave cranially-facingfirst vertebra-engagement surface, an oppositely located concavecaudally-facing second vertebra-engagement surface, a ventral face, adorsal face, and a pair of opposite lateral end faces; (b) a pair offastener receptacles, defined in said spacer body, spaced apartlaterally from each other, each of the fastener receptacles extendingdiagonally from said dorsal face through said spacer body and through arespective one of said lateral end faces, each of the fastenerreceptacles being adapted to receive one of a pair of bone-engagementfasteners extending therethrough and to support said one of saidfasteners further extending diagonally laterally outward from saidrespective one of said lateral end faces of said spacer body, thefastener receptacles being located in the spacer body so as to directeach of the fasteners through respective adjacent articulating processesof a respective facet joint of a pair of adjacent vertebrae whereby eachof the fasteners can immobilize the respective facet joint when saidimplantable device is implanted between those adjacent vertebrae.
 2. Theimplantable device of claim 1 wherein said ventral face defines anengagement ledge, said engagement ledge being adapted to fit against adorsal surface of a lamina of a cranial one of said vertebrae afterhemilaminotomy of said cranial one of said vertebrae.
 3. The implantabledevice of claim 1 including a fastener-locking device located in atleast one of said fastener receptacles in said spacer body.
 4. Theimplantable device of claim 1 wherein said fasteners are bone screws,and wherein each said fastener receptacle includes a locking deviceengaging a screw head of one of said bone screws so as to retain saidbone screw in a selected relationship to said spacer body.
 5. Theimplantable device of claim 1 wherein said first vertebra-engagementsurface includes a plurality of ridges oriented parallel with saiddorsal surface.
 6. The implantable device of claim 1 wherein at leastone of said first and second vertebra-engagement surfaces has a surfaceconfiguration including projections.
 7. The implantable device of claim1 wherein said ventral face includes a recessed portion adjacent to oneof said first and second vertebra-engagement surfaces.
 8. Theimplantable device of claim 1 wherein each of said first and secondvertebra-engagement surfaces extends from said ventral face to saiddorsal face.
 9. An implantable device for use in fusing a pair ofvertebrae together comprising: (a) an interspinous spacer body includinga narrow central portion having a first transverse dimension and havinga pair of lateral ends each having a parallel, second transversedimension, larger than the first transverse dimension, the bodyincluding: (i) a first, cranially-facing, vertebra-engaging face; (ii) asecond, caudally-facing, vertebra-engaging face, said first and secondvertebra-engaging faces being generally concave and facing oppositelyapart from each other and extending from the narrow central portiontoward the lateral ends; (iii) a ventral side having aventrally-oriented face adjacent the first vertebra-engaging face, saidventrally-oriented face including a recessed portion shaped to bearagainst a dorsal aspect of a vertebral surface, and a lower portionprotruding a distance in a ventral direction relative to said recessedportion and being of a size to fit within a cavity defined betweeninferior articulating processes of a vertebra after hemilaminotomy; (iv)a dorsal face; (v) a pair of opposite lateral end faces; and (vi) a pairof fastener receptacles, each said fastener receptacle extending fromsaid dorsal face through said spacer body diagonally to a respective oneof said opposite lateral end faces; and (b) a pair of fasteners eachengaged in a respective one of said receptacles and extending laterallyoutwardly at an acute angle with respect to said dorsal face, and in aventrally inclined direction from a respective one of said oppositelateral end faces, far enough to engage and fasten together a respectivefacet joint between a pair of vertebrae.
 10. The implantable device ofclaim 9 including a fastener locking device in one of said fastenerreceptacles.
 11. The implantable device of claim 9 wherein saidvertebra-engaging faces are oriented oppositely and each is located soas to confront a respective spinous process when said device isimplanted.
 12. A biologically implantable device for use in fusingtogether a pair of adjacent vertebrae, the device comprising: (a) abiologically implantable spacer body having a concave firstvertebra-engagement surface, an oppositely oriented concave secondvertebra-engagement surface, a ventral face, a dorsal face, and a pairof opposite lateral end faces; (b) a pair of fastener receptaclesdefined in said spacer body, each extending from said dorsal facethrough said spacer body and through a respective one of said lateralend faces, each said fastener receptacle being located and directed soas to receive one of a pair of bone-engagement fasteners extendingdiagonally therethrough generally laterally and ventrally from saidrespective one of said lateral end faces of said spacer body.